Changning-Menglian Belt Research Articles

Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo‐Tethyan Ocean

AbstractThe Changning–Menglian Belt represents the main Paleo‐Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana‐ and Eurasia‐derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo‐Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low‐Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine‐spinel Fe‐Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle‐melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)‐and primitive‐mantle‐normalized multi‐element patterns with slightly positive Nb‐Ta anomalies, but negligible Eu and Zr‐Hf anomalies. The volcanics have similar Sr‐Nd‐Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)‐like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB‐related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed‐system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB‐type mantle source (i.e., asthenospheric mantle) containing garnet‐spinel lherzolite material. Combing this new data with that from oceanic seamount volcano‐sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward‐migrating microcontinent of Lanping–Simao.

Construction mode of a Middle Permian sponge reef in theChangning–MenglianBelt, western Yunnan, China

The Changning–Menglian Belt in western Yunnan, China has long been acknowledged as a major suture zone of the Palaeo‐Tethys in south‐east Asia. Although bearing a wealth of environmental information for this vanished ocean, the Permo‐Carboniferous limestones of seamount resemblance in this belt have only been sketchily studied heretofore in terms of their sedimentary features. This study presents a detailed investigation on a Middle PermianArchaeolithoporella‐microbial‐sponge reef in the Damingshan Formation of these limestones. This reef could be ascendingly subdivided into five parts according to its petrographic and sedimentolotgic properties. The first, third, and fifth parts are characterized by the mixture ofArchaeolithoporella‐sponge boundstone, sponge cementstone, microbial boundstone, and skeletal wackestone. They are interpreted to be constructed in a moderate‐energy environment through the following processes: sponges in clusters were partly encrusted byArchaeolithoporella, and mostly toppled over after death; whereafter, spaces within frameworks formed by sponges and/orArchaeolithoporellawere occluded by syndepositional cements; during the processes, sponge clusters were bound together by microbial domes, and both acted as bafflers that trapped sediments. In contrast, the construction of the second and fourth parts occurred in a high‐energy environment and is almost devoid of microbial contribution: sponges that were encrusted byArchaeolithoporellatoppled over posthumously, and then spaces among them were filled by syndepositional cements, ultimately formingArchaeolithoporella‐sponge boundstone. The construction mode of the studied reef, especially sponge toppling and microbial domal frameworks in‐between sponge clusters, differs from that of most coeval sponge reefs, in which the framework is characterized by erect sponges and microbial encrustations onArchaeolithoporella. Our work not only enriches the sedimentary data of limestones in the Changning–Menglian Belt, but also would hopefully deepen the understanding of the growth model of Permian sponge reefs.

Sedimentology and detrital zircon geochronology of the Nanpihe Formation in the central zone of the Changning–Menglian Belt in western Yunnan, China: revealing an allochthon emplaced during the closure of Paleo-Tethys

Sedimentology and detrital zircon geochronology of the Nanpihe Formation in the central zone of the Changning–Menglian Belt in western Yunnan, China: revealing an allochthon emplaced during the closure of Paleo-Tethys

Petrogenesis and tectonic significance of mafic–ultramafic rocks from Southwest Yunnan, China

This study focuses on the mafic–ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (T p ) of these picrites is higher than the T p range of mid-ocean ridges and reaches that of mantle plumes. According to the geochemical characteristics, the Type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by ambient depleted mantle, whereas the Type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic–ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.

Early Carboniferous ammonoids from the Nanduan Formation in the Changning-Menglian Belt, western Yunnan, China

The Changning-Menglian Belt in western Yunnan, China has been interpreted as a remnant of the Paleo-Tethys. The Nanduan Formation, exposed in the eastern part of this belt, is a prominent siliciclastic succession with a thickness of more than 2000m. The age of this formation has remained poorly constrained due to the paucity of fossils. Ammonoids have been previously reported from this formation, but were neither systematically described nor properly illustrated. This study provides descriptions of ammonoids from two beds (Bed-2 and Bed-14) of the Banjiao section in the upper part of the Nanduan Formation. Specimens from Bed-2 preserve diagnostic suture patterns, facilitating generic determination, and thus five taxa are recognized, including Syngastrioceras sp., Somoholites sp., Epicanites sp., Stenopronorites sp., and Prolecanites? sp. Associated indeterminate small conchs from Bed-2 probably represent juveniles. Known ranges of these genera, and in particular, their co-occurrence, indicate a late Mississippian, and most probably a Serpukhovian age for the upper part of the Nanduan Formation. Only a single taxon, Fayettevillea sp., has been recognized from Bed-14, which also supports the Serpukhovian age.

Petrogenesis and tectonic significance of mafic-ultramafic rocks from southwestern Yunnan, China

This study focuses on the mafic-ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (Tp) of these picrites is higher than the Tp range of mid-ocean ridges and reaches that of mantle plumes. According to their geochemical characteristics, type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by the ambient depleted mantle, whereas type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic-ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.

Sedimentology and detrital zircon geochronology of the Nanduan Formation (Carboniferous) of the Changning-Menglian Belt: indications for the evolution of Paleo-Tethys in western Yunnan, China

The Changning-Menglian Belt in western Yunnan, China, has been considered for decades as a remnant of Paleo-Tethys in this region. However, the understanding of the geological evolution of the belt remains sketchy and incomplete. As a significant component of the stratigraphic successions in the eastern part of the belt, the siliciclastic Nanduan Formation preserved essential information of the developing history of the belt. Petrologic studies of the formation show that it was deposited in a neritic environment. The increase of mudstone beds and decrease of grain size of sandstones upwards suggest a gradual deepening of sedimentary environment. Petrographic and geochemical characteristics of sandstones show that they were derived from mature continental provenance and deposited in a passive continental margin setting. Detrital zircon ages of the formation range from 362 to 3685 Ma. The youngest detrital zircon age (362.2 ± 5.5 Ma) confines the deposition age of the sandstones to being younger than the latest Devonian. Dominant age populations of ca. 950 Ma and ca. 550 Ma can be correlated with the magmatic events that occurred in Eastern Ghats-northern Prince Charles (India–Antarctica) (ca. 950 Ma) and Pan-African orogens (Australia–India) (ca. 550 Ma), which are considered, therefore, to be the main provenance of the Nanduan Formation. These results let us consider that the Nanduan Formation and the underlying Lancang Group were formed on the margin of the Baoshan-Shan Block (therefore, autochthonous), and the Carboniferous‒Permian limestones of seamount resemblance, the oceanic bedded cherts, and basic and ultra-basic igneous rocks in the central zone of the belt are overthrusted slices (allochthonous).

Petrology, geochemistry and metamorphic evolution of Lancang Group in the Changning-Menglian complex belt and its implications on the tectonic evolution of the Paleo-Tethys

云南三江地区昌宁-孟连杂岩带中的澜沧岩群内出露多种类型变沉积岩，主要包括含十字蓝晶石榴云母片岩、石榴云母片岩、硬绿泥石白云母片岩和绿泥蓝闪钠长片岩等。系统的岩相学观察、矿物化学和相平衡模拟研究结果表明，不同类型的变沉积岩保存了不同的变质演化历史。含十字蓝晶石榴云母片岩记录了自中温榴辉岩相降温降压至角闪岩相的退变质过程，峰期矿物组合为石榴子石+蓝晶石+多硅白云母+硬玉，变质温压条件为600~750℃和19~30kbar。通过石榴子石的XPrp和XGrs，和多硅白云母的Si值限定石榴云母片岩的峰期矿物组合包括石榴子石+多硅白云母+绿辉石+硬柱石+钠云母，峰期温压条件为430~475℃和17~19.5kbar。硬绿泥石白云母片岩矿物组成包括硬绿泥石+多硅白云母+钠云母，然而峰期矿物组合则以多硅白云母+钠云母+纤柱石为特征，通过多硅白云母的Si值限定峰期温压条件约为300~330℃和17~19kbar。石榴云母片岩和硬绿泥石白云母片岩较为一致地记录了从峰期硬柱石蓝片岩相升温降压至绿帘石蓝片岩相的退变质过程。绿泥蓝闪钠长片岩主要通过变质反应和成因矿物学特征大致估算其温压条件约为430~520℃和9~11kbar。岩石地球化学性质研究结果表明，这些变沉积岩的平均化学成分与大陆岛弧、活动大陆边缘和上地壳沉积物成分较为接近，其原岩主要为一套成熟度较差的泥砂质岩类和少量的中基性火山岩。该泥砂质原岩的沉积物源以中酸性岩浆岩为主，存在不同程度古老沉积物的加入。构造判别图解表明，研究区样品主要形成于大陆岛弧或活动大陆边缘的构造背景。变质作用特征和原岩地球化学属性综合分析表明，昌宁-孟连杂岩带内大面积分布的澜沧岩群均发生不同程度的俯冲-消减过程，表现出多样的变质演化P-T轨迹的特征，表明澜沧岩群经历了多期/多阶段复杂的构造演化历史。

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

Middle Triassic radiolarians from cherts/siliceous shales in an extensional basin in the Sukhothai fold belt, Northern Thailand

More than thirty species of radiolarians are described from maroon chert and siliceous shales from the Den Chai and Santisuk sections in the east of the Sukhothai fold belt. The fauna obtained from Den Chai comprises Muelleritortis cochleata cochleata, M. cochleata tumidospina, M. expansa, Triassocampe deweveri, T. coronata, T. scalaris, Annulotriassocampe companilis, A. multisegmantatus, A. sulovensis, Pseudostylosphaera coccostyla coccostyla, P. imperspicua, Canoptum inornatus, C. levis, Corum kraineri, Spongoserrula rarauana, Orbiculiforma karnica and others. The assemblage may be correlated with those reported in the Fang-Chiang Dao and Lumphun areas in the north of Thailand and in the Changning-Menglian belt of western Yunnan, South China and suggests a late Ladinian (Middle Triassic) age. The radiolarian fauna from the Santisuk Section comprises Muelleritortis cochleata cochleata, M. sp., Striatotrissocampe nodosoannulata, Triassocampe. deweveri, T. sp., Pseudogodia? sp., Pseudostylosphaera coccostyla coccostyla, Archaeocenosphera sp., Annulotriassocampe multisegmantis, Pseudostylosphaera sp., and others which indicates a late Ladinian age. Middle Triassic radiolarian chert and siliceous shale from Den Chai and Santisuk are interpreted as having been deposited in an extensional continental margin within the Sukhothai fold belt and not in the Devonian–Permian back-arc basin of the Nan suture. These deep marine sequences were deposited in a subsiding continental margin in the Lampang-Phrae Basin. We consider these sequences to be part of the Hong Hoi Formation (Anisian–Ladinian) of the Lampang Group which, in turn, may be compared to the Choushui Formation and other associated Middle Triassic formations in the Simao Basin of South China.

Geochemistry of Triassic siliceous rocks of the Muyinhe Formation in the Changning-Menglian belt of Southwest China

The Changning-Menglian belt, distributed over southwestern Yunnan Province in Southwest China, contains oceanic rocks that are considered to be remnants of the Paleotethys. This study observed Triassic siliceous rocks of the Muyinhe Formation in the Changning-Menglian belt and analyzed their geochemistry. The samples have high concentrations of SiO2 (81.65 wt.%–88.38 wt.%; average: 84.99 wt.%±2.14 wt.%). Most of the samples were plotted in the non-hydrothermal field on the Al-Fe-Mn diagram. Most of the samples were plotted in the continental margin field on the Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3) and (La/Ce)N-Al2O3/(Al2O3+Fe2O3) diagrams. Moreover, the samples show a flat REE (rare earth element) pattern normalized to NASC (North America shale composite). These geochemical results, in addition to the lack of rhythmical bedding of the siliceous rocks, strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits. Although previous studies have suggested that the siliceous rocks are pelagic deposits, the present results indicate that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.

East flank of the Sibumasu block in NW Thailand and Myanmar and its possible northward continuation into Yunnan: a review and suggested tectono-stratigraphic interpretation

The east flank of the Sibumasu block was a passive continental margin, and in NW Thailand is marked by the absence of the autochthonous Middle Permian–Triassic platform carbonates which are widespread across the rest of Sibumasu further west. Instead, the carbonates are represented by hemipelagic cherts, mudstones and sandstones including turbidites. During the northward drift of Sibumasu, following its Early Permian rifting from Gondwana, an accretionary complex was present where Palaeotethyan pelagic rocks as old as Devonian were subducted beneath the Sukhothai volcanic arc. At the time of Sibumasu’s collision with the Sukhothai arc, beginning in the Middle Triassic, the accretionary complex was thrust westwards across the east flank of Sibumasu. It is suggested that in the Late Triassic the thrust pile which had been the accretionary complex underwent erosion and was the source of terrigenous clastic rocks deposited further west in a foredeep basin.The boundary of Sibumasu’s east flank with the Permo-Triassic carbonate platform further west is the arcuate Mae Ping-Nam Teng Fault system. Notwithstanding later Cenozoic strike-slip displacement, those faults (as well as the Mae Yuam Fault) are interpreted to have had an earlier history of westward-directed Indosinian thrusting.Northwards in Myanmar and Yunnan the Sibumasu Permo-Triassic carbonate shelf continues as the Shan Plateau and Baoshan Block. The east flank is represented by the Changning–Menglian Belt, and the Palaeotethys ‘cryptic suture’ in Thailand possibly joins with the Lancangjiang Suture.

Tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the Sanjiang region, SW China

The Sanjiang region in SE Tibet Plateau and NW Yunnan is known to have formed by amalgamation of Gondwana-derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from Paleozoic to Mesozoic. In this paper we provide a synthesis of tectonic evolution, magmatism and metallogeny in the region based on data from literatures. Early Paleozoic ophiolites (473–439Ma) in the Changning–Menglian belt indicate the existence of a Proto-Tethys ocean in this region. Two episodes of subduction-related magmatism in the early-Paleozoic, one occurred in the Baoshan and Tengchong blocks at 502–455Ma and the other occurred in the Simao block at 421–401Ma, are regarded as evidence for two different events of subduction of the Proto-Tethys ocean at different locations. The Proto-Tethys was succeeded in early-Devonian by the Paleo-Tethys which comprised the main ocean and three branches: Ailaoshan, Jinshajiang and Garzê–Litang. The Changning–Menglian main ocean existed from middle-Devonian to middle-Triassic. The remnants of the oceanic crust are preserved in a few places in the Longmu Tso–Shuanghu suture as well as in the Changning–Menglian ophiolite belt. The eastward subduction of the main oceanic plate from early-Permian to early-Triassic formed a prominent arc terrane stretching >1500km from Yunnan to eastern Tibet. From the waning stage of subduction to post-subduction, numerous S-type granite plutons with ages varying between 230 and 219Ma, such as the Lincang batholith in Yunnan were emplaced at or close to the suture. This event produced several hydrothermal W–Sn deposits in the region. The tectonic evolution and associated magmatism of the Jinshajiang and Ailaoshan branch oceans are generally comparable to those of the main ocean. However, the branch oceans were subducted westward instead. The Garzê–Litang branch ocean also underwent westward subduction from middle-Devonian to late-Triassic. Arc-related high Sr/Y porphyry intrusions and associated porphyry-skarn Cu–Mo–Au deposits are common in the Jinshajiang–Ailaoshan region, especially in the Yidun arc which formed prior to Jurassic. The VMS deposits in the Sanjiang region formed in diverse tectonic settings including middle-Silurian back-arc basins, Carboniferous oceanic islands, Paleozoic subduction zones and Triassic post-subduction rifting environments. The Mesozoic and early-Cenozoic evolution of the Baoshan and Tengchong blocks was largely influenced by eastward oceanic subduction of the Meso- and Neo-Tethys from late-Permian to middle-Cretaceous and from late-Cretaceous to ~50Ma, respectively. Abundant early-Cretaceous granitoids and associated skarn-type Pb–Zn and Sn–Fe deposits in the Baoshan and Tengchong blocks were produced in the background of the Shan boundary oceanic slab subduction to the west and the break-off of the Nujiang-Bitu oceanic slab to the north. The subduction of the Neo-Tethys oceanic plate beneath the Tengchong block from Late Cretaceous to Paleogene formed abundant S-type granitoids and many skarn-type and greisen-type Sn–W deposits. Granitoids formed at 105 to 81Ma and contemporaneous hydrothermal W, Mo, Ag and Au deposits, which temporally coincided with the subduction of the Neo-Tethys, are common in the Yidun arc terrane.

Middle Triassic radiolarian faunas from Chiang Dao, Northern Thailand

Middle Triassic radiolarians were obtained from a bedded chert sequence in Chiang Dao, northern Thailand. Sixty-four species were identified into two assemblages, which indicate two stratigraphic levels including lower Illyrian (lowermost upper Anisian) and upper Illyrian (uppermost Anisian). The occurrence of upper Anisian radiolarians in our study indicates the former existence of a relatively deep marine environment in northern Thailand with a seaway between eastern and western Palaeotethys. The stratigraphic units containing bedded chert in this area were formerly dated on the basis of fossil evidence and petrological similarities. However, the strata in the study area are possibly not part of a continuous stratigraphic sequence, but consist of tectonic slices. The fossil age from any slice indicates only the geological age of the given slice, and adjacent slices are often of different ages. Parts of the sequence formerly mapped as Carboniferous and Permian are Middle Triassic in the study area. The sequence covers the Carboniferous to Middle Triassic and can be correlated with the eastern zone of the Changning-Menglian Belt in southwestern Yunnan, China.

Sedimentary facies of Carboniferous–Permian mid‐oceanic carbonates in the Changning–Menglian Belt, West Yunnan, Southwest China: Origin and depositional process

AbstractHuge carbonate rock bodies ranging in age from the Visean (Middle Mississippian/Early Carboniferous) to the Changhsingian (Lopingian/Late Permian) overlie a basaltic basement in the Changning–Menglian Belt, West Yunnan, Southwest China. These carbonates lack intercalations of terrigenous siliciclastic material throughout. These lines of evidence indicate that they formed upon an isolated and continuously subsiding mid‐oceanic island (or plateau), probably of hotspot origin. The carbonates are grouped into a shallow‐water carbonate platform facies regime observed in the Yutangzhai section and a relatively deep‐water carbonate slope facies regime typically represented in the Longdong section. These two facies regimes developed contemporaneously as parts of a carbonate depositional system on and around a mid‐oceanic volcanic edifice. The carbonate platform is subdivided into four facies, including platform‐margin, shoal, lagoon, and peritidal facies. Along the measured Yutangzhai section of the platform facies regime, the vertical facies succession from the platform‐margin facies into inner‐platform facies such as the shoal and lagoon facies is recognized. This facies succession is explained as resulting from the progradation of the carbonate platform. Worm tubes occur as a main reef builder in platform‐margin facies of the Mississippian. Their occurrence as major constituents in a high‐wave‐energy reef is peculiar to Carboniferous reef distributions of the world. The occurrences of other reef‐ and/or mound‐building organisms and peritidal dolo‐mudstone are almost consistent in timing with those of Panthalassan counterparts such as the Akiyoshi and Omi limestones of Japan, and probably exhibit the worldwide trend.

Lopingian (Late Permian) foraminiferal faunal succession of a Paleo‐Tethyan mid‐oceanic carbonate buildup: Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China

AbstractThis paper deals with a Lopingian (Late Permian) foraminiferal faunal succession of the Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China, which has been geologically interpreted as one of the closed remnants in East Asia of the Paleo‐Tethys Ocean. The Shifodong Formation is the uppermost stratigraphic unit in thick Carboniferous–Permian carbonates of the belt. These carbonates rest upon bases consisting of oceanic island basalt and are widely accepted as having a Paleo‐Tethyan mid‐oceanic (seamount‐ or oceanic plateau‐top) origin. Sixteen taxa of fusuline foraminifers and 37 taxa of smaller (non‐fusuline) foraminifers are recognized from the type section of the Shifodong Formation located in the Gengma area of the northern part of the Changning–Menglian Belt. Based on their stratigraphic distribution, three fusuline zones can be established in this section: they are, in ascending order, the Codonofusiella cf. C. kwangsiana Zone, Palaeofusulina minima Zone, and Palaeofusulina sinensis Zone. These three biozones are respectively referable to the Wuchiapingian, early Changhsingian, and late Changhsingian, of which the Wuchiapingian is first recognized in this study in the Changning–Menglian mid‐oceanic carbonates. The present study clearly demonstrates that the foraminiferal fauna in a Paleo‐Tethyan pelagic shallow‐marine environment still maintained high faunal diversity throughout the almost entire Lopingian, although the very latest Permian fauna in the upper part of the Palaeofusulina sinensis Zone of the Shifodong section records a sudden decrease in both faunal diversity and abundance. Moreover, the Shifodong faunas are comparable in diversity with those observed in circum‐Tethyan shelves such as South China. The present Paleo‐Tethyan mid‐oceanic foraminiferal faunas are definitely more diversified than coeval mid‐oceanic Panthalassan faunas, which are typically represented by those from the Kamura Limestone in a Jurassic accretionary complex of Southwest Japan. It is suggestive that the Paleo‐Tethyan mid‐oceanic buildups presumably supplied a peculiarly hospitable habitat for foraminiferal faunal development in a pelagic paleo‐equatorial condition.

The Permian seamount stratigraphic sequence in Chiang Mai, North Thailand and its tectogeographic significance

The widespread Permian carbonate strata outcropped in northwestern Thailand are considered as the evidence for the Late Paleozoic shallow Tethys. Our investigation, however, shows that basalt can be discovered usually under the Permian carbonate sequence in Chiang Mai-Fang area, northwestern Thailand. The basalt belongs to subalkalic basalt and potassic trachybasalt. They are characterized by high P and Ti in major elements, by high content, enriching LREE, lacking δEu anomaly in rare earth elements, and by enrichment of the large ion lithophile element (LILE) (K, Rb, Ba) and high field strength element (HFSE) (Nb, Ta, Zr) in trace elements, which can be compared with the characters of the oceanic island basalt in Three Rivers (Jinsha River, Lancang River, and Nujiang River) area, southwestern China. Therefore, the Permian carbonate in the studied area was deposited on a seamount, rather than on a stable carbonate platform. The oceanic basin is correlated to the Late Paleozoic ocean represented by the Changning-Menglian Belt in southwestern China and they are a major basin of the Paleo-Tethyan Archipelagoes Ocean. The result indicates nonexistence of a Shan-Thai Block in the Late Paleozoic.

The Lancang River Zone of southwestern Yunnan, China: A questionable location for the active continental margin of Paleotethys

In an attempt to constrain a Late Paleozoic tectono-metamorphic event along the Lancang River Zone, fourteen samples were processed for K/Ar dating on fine mineral fractions and detrital muscovites from this zone in southwestern Yunnan, China. The samples include mica schists, mylonites and gneisses from the Proterozoic Lancang Group and phyllites from the western part of the Simao Basin. In addition, one Ar/Ar analysis was performed on separated phengites from a blueschist of the central part of the Lancang Group. The results reveal a considerable spread of ages; the tectonic evolution of the zone is constrained by the new data, which accentuate two temporally separate, but spatially overlapping events: (i) a Late Carboniferous high-P/low-T metamorphism related to an east-vergent, Late Paleozoic thrust belt, inverting a Devonian to Carboniferous marginal basin of the Yangtze-Platform, and (ii) an upper Permian and Triassic low-P/high-T belt caused by a post-orogenic stage of rifting with distinct petrological and geochemical similarities of the igneous rocks to the Emeishan Large Igneous Province. These results imply that no active continental margin accounts for the subduction of the Paleotethys main branch, proposed to be recorded either along the Lancang River or the Changning-Menglian Belt.

A study on the geochemical characteristics of Upper Permian continental marginal arc volcanic rocks in the northern segment of South Lancangjiang Belt

Geochemical characteristics of the Upper Permian (P2) continental marginal arc volcanic rocks are described, which have been found recently around the areas of Xiaodingxi and Zangli on the eastern side of the Yunxian-Lincang granite, in terms of rock assemblage, petrochemistry, REE, trace elements, Pb isotopes, geotectonic environment and so on. The volcanic rock assemblage is dominated by basalt-andesite-dacite, with minor trachyte andecite-trachyte; the volcanic rock series is predominated by the calc-alkaline series, with minor tholleiite series and alkaline series rocks ; the volcanic rocks are characterized by high A12O3 and low TiO2, with K2O contents showing extremely strong polarity; the REE distribution patterns are characterized by LREE enrichment and right-inclined type; trace elements and large cation elements are highly enriched, Ti and Cr are depleted, and P and Nb are partially depleted; the Pb composition is of the Gondwana type; the petrochemical points mostly fall within the field of island-arc volcanic rocks, in consistency with the projection of data points of continental marginal volcanic rocks in the southern segment of the South Lancangjiang Belt and the North Lancangjiang Belt. This continental marginal arc volcanic rock belt, together with the ocean-ridge and ocean-island volcanic rocks and ophiolites in the Changning-Menglian Belt, constitute the ocean-ridge volcanic rock, ophiolite-arc rock-magmatic rock belts which are distributed in pairs, indicating that the Lancangjiang oceanic crust subducted eastwards. This result is of great importance in constraining the evolution of the paleo-Tethys in the Lancangjiang Belt.

Devonian to Triassic Successions of the Changning‐Menglian Belt, Western Yunnan, China

Abstract Phanerozoic strata are distributed in several north‐south trending zones in the central part of the Changning‐Menglian Belt. Four types of Devonian to Triassic stratigraphic successions can be identified: (1) clastics with limestone lenses in the mid‐section, changing up‐section into alternations of fine clastics and cherts; (2) clastics with chert intercalations and limestone lenses, and topped by Permian basic volcanics; (3) clastics‐basic volcanics—carbonates—clastics; and (4) limestones, dolomitic limestones—dark gray thin‐bedded limestones, argillaceous limestones, mudstones and siliceous mudstones. Devonian to Triassic cherts occur in different horizons and different zones from east to west. These cherts are usually transitional to their neighboring clastics. There is no continuous Devonian to Middle Triassic chert sequence in the central zone of the Changning‐Menglian Belt as Liu et al. (1991, 1993) reported. Volcanics and the overlying carbonates described by some workers as “seamount” sequences are more likely to have formed in a marine environment on continental crust. Succession (4) is newly recognized in the area from Menglai to Yong'an in Cangyuan County and further north to Padi of Gengma County. Basalts, cherts and clastics also appear in this area. Mid‐Triassic (very probably Ladinian) radiolarians extracted from bedded cherts in the Ganzhejidi section indicate that they are in higher stratigraphic positions. The change from bedded cherts via siltstones to thick‐bedded sandstones with thin‐bedded fine intercalations in the Ganzhejidi section and (some other outcrops along the road from Cangyuan to Gengma) suggests a fundamental change of sedimentary environment caused by a rapid increase of a large quantity of detritus supply. These siliciclastic sediments are possibly syn‐orogenic deposits.